High [CVE-2026-53151] Fix the ACK parser to extract the SACK table for parsing

In the Linux kernel, the following vulnerability has been resolved: rxrpc: Fix the ACK parser to extract the SACK table for parsing Fix modification of the received skbuff in rxrpc_input_soft_acks() and a potential incorrect access of the buffer in a fragmented UDP packet (the packet would probably have to be deliberately pre-generated as fragmented) when AF_RXRPC tries to extract the contents of the SACK table by copying out the contents of the SACK table into a buffer before attempting to parse AF_RXRPC assumes that it can just call skb_condense() and then validly access the SACK table from skb->data and that it will be a flat buffer - but skb_condense() can silently fail to do anything under some circumstances.

Note that whilst rxrpc_input_soft_acks() should be able to parse extended ACKs, the rest of AF_RXRPC doesn't currently support that. Further, there's then no need to call skb_condense() in rxrpc_input_ack(), so don't.

An attacker could potentially craft a fragmented UDP packet to trigger an incorrect buffer access within the kernel, which may lead to system instability or other undefined behavior.

However, fragmented UDP packets may use non-linear sk_buffs, and skb_condense() may fail to linearize them, potentially causing out-of-bounds memory access, incorrect ACK processing, or a kernel crash.